1. Field of the Invention
The present invention relates to a method of manufacturing a monolithic ceramic capacitor which comprises a ceramic laminate body having a plurality of inner electrodes buried therein in layers, and outer electrodes provided on both end surfaces of the ceramic body, and separated from the ceramic body by conductive ceramic films.
2. Description of the Background Art
A monolithic ceramic capacitor generally comprises a ceramic laminate body which is formed by alternately stacking dielectric ceramic layers and inner electrodes, and further comprises outer electrodes, which are mainly composed of silver, formed on both end surfaces of the ceramic laminate body, and electrically connected to the inner electrodes.
In general, solder is used to connect such a monolithic ceramic capacitor to an interconnection substrate such as a printed circuit board, the solder being adhered onto the outer electrodes to attain connection with the interconnection substrate.
However, it has been recognized by a reliability test that the electrical properties of the monolithic ceramic capacitor are impaired by soldering due to various causes including: movement of a metal, particularly tin, contained in molten solder toward the inner electrode layers; electrochemical reaction; and the like. That is, solder is mainly prepared from tin, which is generally easily alloyed with various metals, whereby the use of solder causes a phenomenon wherein an intermetallic compound is formed with the outer electrodes. Part of such an intermetallic compound gradually diffuses into the monolithic ceramic capacitor with time, through diffusion paths of the inner electrodes, or interfaces between the inner electrodes and the dielectric ceramic layers. This causes the electrical properties, such as insulation resistance, for example, of the capacitor to be extremely reduced. Accordingly, it would be desirable to develop a highly reliable monolithic ceramic capacitor, which can prevent any of the elements which make up the solder (when solder is to be solder employed for connecting the capacitor to an interconnection substrate) from diffusion into or around the inner electrodes, thereby to prevent deterioration of the electrical properties of the capacitor.
U.S. Pat. No. 4,604,676 in the name of the assignee discloses a monolithic ceramic capacitor which has conductive ceramic films (conductive metal oxide layers) formed between both end surfaces of a ceramic laminate body and outer electrodes. According to this '676 patent, the conductive ceramic films are adapted to prevent deterioration of electrical properties, particularly insulation resistance, by preventing reduction of the dielectric ceramic material caused by the outer electrodes. The presence of such conductive ceramic films also prevents the elements in solder from diffusion into or around the inner electrodes.
That is, due to interposition of the conductive ceramic films, the inner and outer electrodes, which are electrically connected with each other, are structurally separated by conductive ceramic films having a low metal diffusion coefficient. Thus, even if there is a chance for the solder to be in contact with the outer electrodes, the elements, mainly tin, forming the solder, are prevented from diffusion into or around the inner electrodes. Thus, the electrical properties of the laminated ceramic capacitor are stably maintained.
However, the aforementioned '676 patent discloses only the use of a thin film forming method, such as sputtering, as a method of forming the conductive ceramic films. In order to form the conductive ceramic films in specific regions such as the surface parts of the ceramic laminate body, in order to expose the inner electrodes, masks as well as high-priced equipment such as a sputtering apparatus are required. Further, a relatively long time is required to form the conductive ceramic films. Thus, the overall cost of obtaining this prior art monolithic ceramic capacitor is increased.